The invention relates generally to formation evaluation using borehole sonic logging. More specifically, the invention relates to a method for distinguishing between intrinsic and stress-induced anisotropy in an anisotropic formation.
A formation is said to be anisotropic if the value of a property of the formation varies with direction of measurement. A formation has shear wave anisotropy if shear wave velocity in the formation varies with azimuth. Shear wave anisotropy can be detected in a formation using a crossed-dipole sonic log obtained from a borehole penetrating the formation. The crossed-dipole sonic log is generated by measuring velocities of two orthogonal dipole modes in the formation. Two forms of shear wave anisotropy are considered herein: intrinsic and stress-induced. Intrinsic shear wave anisotropy may arise from intrinsic structural effects, such as layering of shale in a deviated borehole or aligned fractures, and tectonic stresses. Stress-induced shear wave anisotropy arises from the redistribution of the far-field horizontal stresses around the borehole. Existing crossed-dipole sonic log indicates anisotropic zones of the formation but does not indicate the dominant underlying cause of the anisotropy. However, distinguishing between intrinsic and stress-induced anisotropy is important. Intrinsic anisotropy, specifically fracture anisotropy, plays an important role in drilling, production, and completion strategies. For example, it is desirable that boreholes are placed in the formation such that they intersect as many fractures as possible. Stress-induced anisotropy plays an important role in completion strategies. For example, perforations oriented perpendicular to minimum stress direction can be used to optimize hydraulic fracture design.
Plona et al. describe a method for distinguishing between intrinsic and stress-induced anisotropy in a formation using a crossed-dipole sonic log. (Plona T. J., et al., “Using Acoustic Anisotropy,” paper presented at 41st SPWLA Symposium: June 2000). The method exploits the fact that stress-induced dipole anisotropy in slow formations exhibits flexural mode dispersion crossover whereas intrinsic dipole anisotropy does not. (Plona T. J., et al., “Stress-Induced Dipole Anisotropy: Theory, Experiment and Field Data,” paper RR, presented at 40th SPWLA Symposium '99). The method includes obtaining crossed-dipole waveforms from a borehole. Alford Rotation is applied to the crossed-dipole waveforms to identify the fast-shear direction. Flexural dispersion curves, i.e., slowness versus frequency curves, are obtained by processing the rotated waveforms for dipole polarizations parallel and normal to the fast-shear and slow-shear directions using a modified matrix pencil algorithm. The slow-shear direction is orthogonal to the fast-shear direction. Slowness, measured in microseconds per foot, is the amount of time for a wave to travel a certain distance. FIGS. 1A and 1B show dispersion curves for an intrinsic anisotropic formation and a stress-induced anisotropic formation, respectively. The dispersion curves are generally parallel for an intrinsic anisotropic formation and cross for a stress-induced anisotropic formation. Although not shown in the figures, dispersion curves coincide for an isotropic formation.
The Plona et al. method of distinguishing between intrinsic and stress-induced anisotropy requires interpretation of individual dispersion curves, which may not be efficient or practical for large data sets. A continuous method of distinguishing between intrinsic and stress-induced anisotropy would be useful to efficiently diagnose the cause of anisotropy.